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New bowsprit

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Charles:
So, the bowsprit (stick that pokes out of the front) on my sailing yacht got bust in a gale last winter, so I need to make a new one - woodwork in this post, some machined parts to follow...

First, source some clean straight-grained Douglas fir, I need a finished spar of 115mm diameter, so I'm aiming to make a square blank of 120mm, so that's 4 pieces 120 x 30 x 3.7m long. These pieces were laminated together using epoxy, first, a thinned coat wait 30 minutes or so for it to soak in, then an undiluted coat, stack the piece up and use a vacuum bag to clamp the pieces together. The supporting trestles were carefully spaced while the epoxy cured, so the spar (should) be neither sagging nor hogging...

Once cured, the blank was passed through the thicknesser to bring it down to its finished size of 115mm square.

Next, a slot was cut right through at one end to accept the outhaul sheave, this was simply chain drilled, and cut with a chisel as for a mortice and tenon joint. The axel hole was also drilled at this stage.

Next, I made a spar gauge from a scrap of wood, positioning the holes with the mill to make it accurate, and the holes plum. The lines this tool makes are the edges of a perfect octagon. Next step is to make remove the corners with a power planer to make the square section octagonal. Then using a hand plane make the octagon round, then sandpaper to remove the plane marks.

I reckon this job has now generated 1 entire wheelie bin full of wood shavings...

Charles:
Machined parts to go with the wood work...

At one end of the bowsprit there is a sheave used to pass a rope around which pulls the sail out to the end of the spar...

First the bearing, simply a piece of phosphor bronze, 25mm OD, 20mm ID, part off at 12mm.

The sheave itself started as a piece of 12mm Delrin sheet, roughed out on the bandsaw. A hole was drilled and tapped in the centre to allow mounting on a mandrel, then it was turned down round at 120mm diameter, a rope groove cut into the edge with a homemade form tool, the faces relieved with the same form tool. Next, gripping it on the outer edge the centre was bored to 25mm then the phosphor bronze bushing pressed into the centre using the tailstock. The axle was turned from 316 stainless, an M6 hole was made in each end, this will secure a small plate on each end which will then be screwed to the spar to retain the axel in place.

ieezitin:
Nice... any pictures of your boat..

Anthony..

Pete W.:
Hi there, Charles,

Thank you for your interesting posts.

Please could you post a photograph looking at the spar end-on to show the relative orientation of the growth rings of the four pieces of timber?  How did you choose that orientation?

And I ,too, would like to see pictures of your boat. 

Charles:
Pete / Anthnoy,

Boat pictures below...

one of these is a still from a Round the Island Race promotional video, whole thing here:

Pete, see also a picture of the inboard end - slightly shiny as I have coated the end grain with epoxy. The joints run horizontally when in use. The timber stack was not selected on the basis of much science; in theory, the only load the spar sees is compression, so grain orientation shouldn't matter much. I would have preferred quarter sawn timber, but I had to work with what the timber merchant had in stock, in this case, the 4 pieces came from two boards one of which was slightly lighter & softer than the other, the light board forms the middle two layers leaving the dense ones for the outer. There is a point load where the bowsprit traveller rests, so I would prefer that to bear on the harder timber.

Other than that, the section is massively over spec for the compression load, however, it will inevitably see unfair loads occasionally - dipping the sail into a big wave for instance, stuff like that is impossible to predict or model mathematically, so the only way to allow for it is to massively over-engineer...

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